Guide to Electronic Testing--Component tests

This section deals with the problems and solutions of dealing with discrete
components.

==Detecting thermal drift in resistors==

Thermal drift in resistors is one of the most common and annoying troubles
in all kinds of electronic equipment. It causes such symptoms as “it plays
for an hour and then acts up.” If a carbon resistor changes in value as it
heats up, it changes the circuit characteristics.

The time constant will tell you a lot. If the trouble shows up inside of 15
minutes, it’s probably a result of self-generated heat in a resistor, heat
generated in the resistor because of the current it’s carrying. Heat occurs
in plate-load circuits, voltage-dropping resistors, etc. If the trouble takes
an hour or so to appear, the resis tor is being affected by heat traveling
through the metal chassis or from a hot component close by.

The best test for a suspected resistor is to heat it up artificially and watch
for the trouble to appear. For example, if you had a long-time-constant sync
trouble, you could turn the set on, adjust it for correct operation, and then
apply heat to each of the resistors in the sync circuit. Place the tip of a
soldering iron on the body of each resistor, hold it there for 45 to 60 seconds,
and watch for the sync trouble on the screen. Normal operating temperature
in a TV set is 120 to 130°F. The tip of a soldering iron runs about 600°F,
so don’t hold it on the resistor too long—just long enough to get the resistor
warmer than normal or too hot to hold a fingertip on it. Amazon.com Widgets

If the resistor has a tendency toward thermal drift, this test will reveal
it.

The high-value (6, 8, 10, and 20 M) resistors found in age circuits are frequent
offenders. You can find the guilty resistor by turning on the set, heating
up each resistor in turn, and watching the screen for any sign of the original
trouble. There might be more than one defective resistor in a given circuit,
so check them all.

==Checking and repairing potentiometers==

A potentiometer is simply a manually controlled resistor. The term is often
informally shortened to pot.

The effective resistance of a potentiometer is determined by the position
of a rotating shaft. Fig. 1 illustrates the internal construction of a typical
potentiometer. The two end terminals are connected to either end of a U-shaped
resistive element. The control shaft is connected to a sliding element. Rotating
the shaft moves the slider in one direction or the other along the length of
the resistive element, creating a changing resistance. Let’s assume that the
slider starts off as close as possible to end terminal A and is slowly moved
toward end terminal B. The resistance be tween the center terminal and end
terminal A increases, while the resistance between the center terminal and
terminal B de creases by a like amount. The resistance element can be designed
so that the resistance changes linearly ( Fig. 2) or logarithmically ( Fig.
3), depending on the requirements of the specific application.

A trimpot, or trimmer potentiometer works in much the same way, except the
control shaft can be replaced with a screwdriver adjustment. This adjustment
reduces the size of the component and is more secure for “set-and-forget” calibration
controls.

_ Fig. 1 Inside a potentiometer, a slider is moved over a resistive element.
Terminals; Resistive element

_ Fig. 2 (left) Some potentiometers have a linear response. (right) Fig. 3 Other
potentiometers have a logarithmic response.

You occasionally might encounter a slide pot. This component is basically
the same as a standard potentiometer, except the resistive element’s normal
U-shape has been straightened out into a line. The control shaft does not rotate;
instead, it’s moved back and forth over the straight-line resistive element.
A slide pot is used in applications in which the position of the control shaft
is to be directly visible, instead of indicated by markings on a dial. For
example, slide pots are widely used in audio mixer consoles and graphic equalizers.

Some potentiometers, especially trimpots, have just two terminals, instead
of the more common three just described. In such cases, there simply is no
connection with one of the ends of the resistive element—there is just the
center terminal and one of the end terminals.

A three-terminal potentiometer can easily be used in any application calling
for a two-terminal potentiometer. Simply leave one of the end terminals disconnected
and unused in the circuit. Some technicians prefer to short the unused end
terminal to the center terminal. This method won’t affect the operation of
the potentiometer in any noticeable way.

Testing potentiometers:

Testing a potentiometer is usually fairly simple. You can use the ohmmeter
section of any multimeter. It doesn’t much matter what type of multimeter is
used, as long as it’s reasonably accurate in its coverage of the required resistance
range.

As with any test of resistances, testing a potentiometer in circuit can give
inaccurate readings because of parallel resistances in the circuit. When in
doubt, temporarily break the connection between one of the test terminals of
the component and the circuitry.

On a three-terminal potentiometer, first measure the resistance between the
two end terminals. You should get the potentiometer’s full rated resistance.
The setting of the control shaft should have no effect at all on the resistance
read between the two end terminals. For example, if you are testing a 10K potentiometer,
you should get a reading of 10,000 12, more or less. Don’t be too concerned
about the exact resistance value. Most potentiometers are not precision components
when it comes to their full-scale resistance, nor do they need to be in the
vast majority of practical applications. As long as the resistance reading
you get is in the right ballpark, you can assume that the potentiometer has
passed this test.

The more important test is to monitor the resistance from the center terminal
to one of the end terminals. It usually doesn’t matter which one you use. (Of
course, for a two-terminal potentiometer, there is no choice, since there is
only one end terminal.) If possible, clip the test leads of your ohmmeter in
place on the potentiometer’s terminals to leave your hands free. Slowly and
smoothly rotate the control shaft while watching the ohmmeter’s pointer or
display. As the control shaft is rotated toward the end terminal being used
in the test, the resistance should decrease. Rotating the shaft away from the
monitored end terminals should cause an increase in the resistance.

Rotate the control shaft as smoothly as possible. The resistance reading also
should change smoothly and evenly. (Re member that some potentiometers change
their resistance in a linear fashion, while others have a logarithmic response.)
If the resistance jumps around, jiggles up and down, makes large skips, or
gets “stuck” at a specific value over a few degrees of the shaft’s rotation,
a problem is indicated. Such erratic readings generally indicate that the resistive
element is dirty or pitted, resulting in unreliable operation in the circuit.
In some cases, there might be static or a crunching noise in audio circuits
as the potentiometer’s control shaft is rotated.

Cleaning potentiometers:

Slide pots are particularly prone to getting dirty because their construction
inevitably leaves the resistive element more exposed to external contamination
than in most standard round pots. To cure this problem, spray in some cleaner/lubrication
spray, which is sold by almost any electronics parts store. It’s often called
“tuner cleaner.” It’s an aerosol spray can, with a small straw that can be
connected to the nozzle to finely direct the spray. Direct the spray into any
openings in the body of the potentiometer and then work the control shaft back
and forth over its entire range several times to spread the cleaning fluid
over the entire resistive element. It’s a good idea to wait a couple of minutes
to let the fluid dry, but this is rarely absolutely necessary. For a slide
pot, you can easily insert the spray in the slot for the control slider’s back
and forth path.

When in doubt, clean all potentiometers, just to be on the safe side. Unless
you use a ridiculous amount of spray, doing so will never hurt the circuit
and even if dirt isn’t causing the current symptoms, it’s still likely to clean
up dirt that hasn’t built up to a noticeable problem yet. Cleaning the potentiometers
is a form of preventive maintenance.

A lubricant in the cleaning spray is usually helpful, but a few components
might be damaged by lubrication. When in doubt, use an unlubricated cleaning
spray, even though it won’t always be quite as effective.

If cleaning the potentiometer does not help, the resistive element might be
badly pitted and permanently damaged. Try substituting a potentiometer that
is known to be good. If the problem clears up, discard the original potentiometer
as defective, and solder in a permanent replacement.

Many potentiometers, especially those used as volume controls, come equipped
with a switch mounted on the back of the component. When the control shaft
is rotated past a specific spot (usually near one end of its range of movement),
the switch is activated. Sometimes these potentiometer switches develop problems.
They might not make proper contact, or they might get stuck in one position
or the other, ignoring the rotation of the potentiometer’s control shaft.

Visually inspect the switch mechanism. Sometimes you might see that something
is slightly bent. Use a pair of small needle-nose pliers to move it back into
place. Be very careful not to do more damage.

Cleaner/lubrication spray will often solve poor switch contact problems. Spray
some of the cleaning/lubrication solution into the switch mechanism and turn
it on and off several times. If this doesn’t work, you will probably have to
replace the switch, which often entails replacing the entire potentiometer
unit.

Stuck switches are usually unrepairable. The damage is probably permanent.
It won’t hurt to try to bend some small part back into place within the switching
mechanism (if accessible), or even to try cleaning the switch unit, but don’t
get your hopes up too high, or spend a lot of time at it. You will probably
end up having to replace the defective switch, which often means replacing
the entire switched potentiometer unit.

==Capacitance testing==

There are only two things you need to know about almost any paper, ceramic,
or mica capacitor: Is it open, or is it leaky or shorted? Measuring the capacitance
value is seldom necessary, since this information normally is stamped or color-coded
on the capacitor itself. Capacitors of these types are not likely to shift
in value, so the real question boils down to whether a capacitor is good or
bad.

A capacitor tester is handy for determining whether a capacitor is open. Hook
it up and turn the dial rapidly from one end to the other, past the nominal
value of the capacitor. If the tuning eye on the tester opens at all, the capacitor
is not open.

Make the same check with an ohmmeter for values larger than 0.01 uF by touching
the ohmmeter across the capacitor and watching for the charging kick. (Smaller
values give a charging kick, too, but it’s too small to show on the meter.)

Next, and most important, check the capacitor for leakage. A dead short or
high leakage can be caught with the ohmmeter. If the ohmmeter shows any deflection
on the highest ohms range available, the capacitor is bad. For very critical
applications, such as audio coupling capacitors in vacuum-tube amplifiers,
you need a test instrument that reads very small leakage. Even a leak age of
100 M is enough to cause trouble in a coupling capacitor.

The fastest test for a possibly open capacitor is to bridge an other one across
it. If you suspect oscillation is caused by an open bypass capacitor, For example,
bridge it. If the oscillation stops, you have found the trouble. If a coupling
capacitor is suspected, you can test it in two ways: check for the presence
of signal on both the input and output sides of the capacitor; or bridge another
capacitor across it — if the signal now goes through, the original is open.
If a capacitor opens, it has the same effect as taking the capacitor completely
out of the circuit. So, replace it by bridging, and see if the trouble stops.

There has been a lot of development in capacitance meters in recent years.
Newer devices check for opens and shorts, measure leakage, and display the
capacitance value directly. These de vices and other new types of test equipment
were discussed in section 1.

Electrolytic capacitors:

Electrolytic capacitors, unlike paper capacitors, can change value by drying
up. A dry electrolytic is not dry any more than a dry-cell battery is. If the
electrolyte evaporates in either one, it stops working. A battery dies, and
a capacitor opens completely.

With electrolytics, the best test is again, “How well do they work?” If the
service notes specify that a power supply should have 275 V at the rectifier
output and you find only 90 V to 100 V, the input capacitor is very likely
open. Bridge it with a good one; if the voltage jumps up to normal, that was
the trouble. If the ripple or hum level is given as 0.2 V p-p at the filter
output and your scope shows 10 V to 15 V p-p there, bridge the output capacitor.
If the ripple drops to within the proper limits, that capacitor was open.

For bridging purposes, the test capacitor does not need to be an exact duplicate
of the original. It can be much larger or smaller in value, but it must have
a working voltage able to stand what ever voltage is present in the circuit.
It’s not a good idea to bridge electrolytics in transistor circuits with the
amplifier on. The charging-current surge of the test capacitor can cause a
sharp transient spike in the circuits, which can puncture transistors. So,
to bridge-test transistors sets, turn the set off, clip the test capacitor
in place, and then turn the set on again. With the instant starting of transistor
circuitry, this won’t waste any time.

If you find one unit in a multiple-type electrolytic to be bad, replace the
whole can. Whatever condition existed inside that can to make one unit go out
will eventually cause failure of the rest because they’re all parts of the
same assembly. To avoid an almost sure callback, change the whole thing at
once.

Finding the value of an unknown capacitor without a capacitance meter:

If you don’t have a modern capacitance meter handy, there is still a fairly
quick way to find the value of an unknown capacitor.

Note: This method isn’t accurate unless you have a precise ac voltmeter or
calibrated scope and an accurate test capacitor. The method is handy for finding
the values of those odd mica capacitors that everyone has around and can’t
read the color code on.

Put the capacitor in series with a known capacitor, as shown in Fig. 4, and
apply an ac signal voltage across the two. Measure the voltage across the known
capacitor, then measure the voltage across the unknown capacitor. The voltage
ratio will give you the ratio of the capacitances.

What you’re doing is putting two reactances in series across an ac-voltage
source. The result is a voltage divider that works the same way as two resistors
in series across a dc source. Suppose, for instance, that you put 11 V at 1,000
Hz across a series combination consisting of a .005 uF capacitor and an unknown
capacitor. Sup pose then you measured 10 V across the unknown and 1 V across
the known. The voltage ratio for the unknown and known capacitors is 10:1,
and so is the ratio of reactances. The ratio of capacitances is just the opposite,
or 1:10, because the reactance of a capacitor is inversely related to the capacitance:
As capacitance increases, reactance decreases, and vice versa.

In the example given, the unknown capacitor has a value one-tenth as great
as the known capacitor, or .0005 Re member, a voltage ratio of 10: 1 means
a capacitance ratio of 1 : 10. i you prefer, you can work with reactance values,
reading them from a table and then using the table to find the corresponding
capacitance values.

_ Fig. 4 A capacitor’s value can be approximated with a test hook-up like
this.

Testing capacitors with a scope:

An oscilloscope is another handy tool for checking capacitor problems. Generally
the standard low-capacitance probe is sufficient, although in some cases, a
demodulator probe is useful.

To test the capacitor, adjust the scope for maximum sensitivity on vertical
gain. Then touch the probe tip to the hot side of the capacitor to be tested.
Interpretation of the scope trace depends on the capacitor’s intended application
in the circuit. In most circuits, capacitors are used for bypassing, filtering,
or coupling.

A bypass capacitor is always grounded at one end. Unwanted ac components of
a signal are shunted to ground. In this test, you want to determine that the
ac components have indeed been eliminated from the signal.

A filter capacitor is really very similar to a bypass capacitor, although
it’s usually somewhat larger in value. A filter capacitor’s function is to
shunt unwanted signals, such as ac hum or RF signals, to ground. It’s easy
to determine if these components are present in the scope trace.

A coupling capacitor, or blocking capacitor, is placed in the circuit to pass
an ac signal but to block any dc component in the signal. Check the scope trace
to make sure the ac signal is centered around 0 and is not riding on a DC voltage.

==Testing variable capacitors==

Variable capacitors are not used in modern electronics nearly as often as
they once were, but you still might encounter them from time to time, especially
in older radio tuners. In a typical variable capacitor, a set of movable plates,
or rotors, are moved either directly via a tuning knob, or indirectly with
a dial cord and a system of pulleys. These rotors are intermeshed with a second
set of fixed-position plates, known as stators. The rotors move with respect
to the stators. The air between the two sets of plates acts as the capacitor
dielectric, so the relative positioning of the two sets of plates determines
the effect capacitance of the component at each setting.

Test the resistance from any of the rotor plates to the metal frame of the
variable capacitor. If the circuit is mounted on a metallic chassis, the component
might or might not be insulated from the chassis. You should measure a dead
short, or 0 Ohm.

Disconnect the wires connected to the stator terminals and measure the resistance
from each stator plate to the rotor and/or the unit’s metal frame. You should
get an open circuit reading. The ohmmeter should indicate infinity, or a very,
very high resistance. To be doubly sure, it’s a good idea to make this test
at each extreme of the tuning shaft’s range; that is, once with the plates
fully meshed, and once with them as opened up and far apart as possible. You
also should get an infinity reading between individual stator sections.

In servicing equipment with variable capacitors of this type, you also must
watch out for mechanical defects. It’s usually very easy for one or more of
the capacitor’s plates to get bent. A bent plate can restrict the movement
of the tuning control. It also can create a short circuit between the rotor
section and the stator sections, causing noise or a partial or complete loss
of signal. In some cases, turning the tuning control will have no apparent
effect be cause of such a mechanical short circuit.

These tests are only for open-air types of variable capacitors. They are not
appropriate for some other types of variable capacitors.

==Testing inductors==

A coil, or inductor, is basically a pretty simple component. It’s really nothing
more than a length of wire wound in a coil shape around a core of something.
(In some cases, there is an air core. That is, there is nothing at the center
of the coil except empty air.) This type of component isn’t used in modern
electronics nearly as much as it used to be, but it’s still far from uncommon.
You won’t run into as many coils as capacitors or resistors, but the odds are,
you will run across some in the electronic equipment you are servicing.

The basic unit of inductance is the henry (H), or, more commonly, the millihenry
(mH). One henry equals 1,000 millihenries. Unfortunately, standard electronic
test equipment cannot measure inductance values directly (see the next section
of this section). Fortunately, you can do some crude, but effective, good/bad
tests with an ohmmeter to find most common faults with this type of component.

As with most component resistance readings, you probably will have to disconnect
at least one end of the inductor from the circuit to avoid the effects of parallel
resistances that can confuse the picture. However, this problem is generally
less significant for coils than for other types of electronic components because
the resistance of a typical inductor is so low that it will probably be the
major factor in determining the total effective parallel resistance. Still,
there are exceptions.

There are essentially three basic ways a coil can go bad: It can be open,
there can be a short between windings, or there can be a short to the inductor
coil.

Open coils:

Open coils aren’t very common, but they can happen. An open coil is just a
break in the coiled wire, or more likely, in its leads, which connect it to
the rest of the circuit. If you measure the resistance across the terminals
of a coil and get an infinity (or very high) reading, then you have an open
coil. In some cases, it might be worthwhile to take just a moment or two to
see if you can locate the break visually.

In some cases of a broken lead, you can resolder it without too much trouble.
Otherwise the coil is defective and must be re placed. In most practical servicing
situations, there is little point in spending much time in trying to locate
the actual break, since the odds are that it won’t be reparable.

Shorted turns:

It’s trickier to determine whether or not there are one or more shorted turns
in an inductor. Most practical coils consist of many closely wound turns of
fine wire. If some of the insulation chips off, melts, or is otherwise removed,
the conductive wires of adjacent turns will touch, producing a short circuit,
as illustrated in Fig. 5. The coil will act like it has too few turns, and
therefore, it won’t function properly in the circuit.

If you know the nominal resistance of the coil in question, you can simply
measure the resistance across the coil’s terminals. If you get an abnormally
low resistance reading, then a short within the coil is probable. Replace the
defective inductor.

The nominal resistance of coils often is included in service data or specification
sheets. If it’s not, however, it’s a lot harder to determine what the correct
resistance for the coil should be. Normally, it will be a very low value, since,
as far as dc current (and resistance) is concerned, a coil is nothing but along
length of ordinary wire. A dc signal couldn’t care less that the wire is coiled,
rather than straight. Any conductor has some finite resistance per foot. The
longer the conductor, the lower the resistance. Shorted turns in a coil effectively
shorten the conductor length.

Fig. 5 Shorted turns in a coil effectively make it function like a smaller
inductance. Shorted turns.

If the circuit contains more than one coil, you might compare the resistance
readings across each of them to see if they are reasonably similar. Don’t make
too many assumptions. There is no guarantee that all similar-looking coils
in a circuit have the same value. One might have 100 turns, while another might
have 225 turns, so their resistance readings logically will be quite different.
You can, however, sometimes make some reasonable guesses. For example, let’s
say you have a circuit with four coils that look pretty much alike. When you
measure the resistance across each coil, you get the following readings:

Coil -- Ohms

A 51

B 48

C 11

D 55

With these readings, I would certainly be suspicious of coil C, especially
if the schematic indicates that all four coils should have similar inductance
values.

Shorts in inductor coils

The third common type of defect in an inductor is a short to the core. This
is mechanically similar to the shorted turn discussed earlier. One or more
of the windings in the coil has lost some of its insulation for some reason,
and since the turns of wire are pressed tightly against the coil, there is
an electrical short circuit between the core and the exposed winding(s). Obviously,
this is only a problem in an inductor with a conductive core, such as an iron
core. You won’t encounter this type of problem with an air-core coil, of course.

You can determine if there is a core short by measuring the resistance between
either terminal of the inductor and the core itself. Ideally, this resistance
should be infinite. In most practical cases, you will get a finite, but very
high resistance reading. A typical value will be over 25 M The exact winding
to core resistance depends on the core material and configuration, as well
as the quality of the insulation used in the construction of the inductor.

These same tests also apply to RF chokes, since this type of component is
just a special-purpose coil. The term choke has more to do with the function
than the actual construction of the component itself.

You can test a transformer in a similar way. A transformer is made up of
two (or sometimes more) coils wound on a common core so their electromagnetic
fields interact. Test the primary winding as if it was a simple coil. Then
do the same with the secondary winding(s). Most transformers have just a single
secondary winding, but some have more.

Test each secondary winding separately, but don’t get con fused by center
taps. A center top is an extra terminal to permit circuit connection to a midpoint
on the coil. A center tap is clearly shown in the standard schematic symbol
for a transformer with a center-tapped secondary, as illustrated in Fig. 6.
Fig. 7 shows a transformer with three secondary windings. Notice that one of
these secondaries also has a center tap.

You also should test for electrical isolation between the primary and each
secondary winding of a transformer. Measure the resistance between either end
connection of the primary winding and either end connection of the secondary
winding. (For a trans former with more than one secondary, repeat this test
for each secondary winding.) You should get a very high (nominally infinite)
resistance reading. If you get a resistance reading of less than 20 M-Ohm or
so, suspect trouble. There might be a partial or total short between the windings,
which will interfere with the correct operation of the transformer and could
easily result in some dangerous conditions. A very low reading here is a clear
indication that the transformer should be replaced immediately.

Don’t attempt to operate any electrical equipment with such a defective transformer.
If you’re lucky, there will only be major damage to the rest of the circuitry.
However, there is also a very real possibility of dangerous, and potentially
fatal electrical shocks and/or fire.

One important exception to this last test. A special type of transformer,
known as an autotransformer, uses the same coil for both the primary and the
secondary, as illustrated in the schematic symbol shown in Fig. 8. Obviously,
in this case you inevitably will read a very low resistance between the “primary”
and the “secondary,” since they are just different points along the very same
length of coiled wire.

_ Fig. 7 (above) Some transformers have multiple, isolated secondary windings.
This transformer has three secondaries, one of which is center tapped.

_ Fig. 8 An autotransformer uses the same physical coil as both the primary
winding and the secondary winding.

==Measuring inductance==

The question is often asked, “How can you measure inductance?” The best answer
is, “ You can’t.” Inductance measurements, with common shop equipment, are
a practical impossibility. You can figure it out by spending lots of time and
doing a lot of mathematics, but the best advice we can offer is, “Don’t.”

In practical service work, there’s seldom a need to read the inductance of
a coil or transformer in henrys or millihenrys. You are usually interested
in just one thing: continuity. This is a simple ohmmeter test. All service
data give the dc resistances of coils, and as long as your resistance reading
on an inductor is within 5 percent of the specified value, the inductor is
probably all right.

Only two things that can happen to an inductor: It can open completely (which
is fairly easy to find), or it can develop shorted turns. In power transformers,
etc., shorted turns give a very definite indication: smoke. In other circuits,
such as output trans formers, shorted turns cause a drastic loss of output.
You can locate a shorted turn by elimination tests and power measurements.

Flybacks are a special case; they act more like tuned circuits than ordinary
transformers do. You can test them with a special instrument that connects
the coil into a circuit and makes it oscillate. Read the Q of the coil on a
meter. However, you can check flybacks for shorts by reading the cathode current
of the horizontal-output tube and then disconnecting all loads, such as the
yoke and damper circuits, and reading again. If the cathode current is far
above normal, the flyback is internally shorted. When the loads are disconnected,
the current should drop to about one fourth its normal full-load value.

To get an inductance of any particular value, there’s one easy way: buy it.
You can wind coils all day, trying to get an 8.3 uH choke, but if you call
your distributor, you can have a choke coil with exactly 8.3 uH in a few minutes.
Coil and transformer makers have a tremendous selection of coils in all conceivable
sizes, listed by their inductance, mounting style, etc. By far, the easiest
way to work with inductors is to go buy an exact duplicate when you need it.

==Shorted power transformers==

A common source of problems in electrical circuits is the short circuit. The
trick is in determining just where it is. It’s usually somewhere in the load
circuit, but infrequently, the problem might be in the power transformer. You
could waste hours going through every single component in the load circuit
and tearing your hair out because everything checks out fine, when the problem
is really in the power supply itself. Be aware of this possibility, although
it’s definitely the exception, rather than the rule.

A handy “quick-and-dirty” test is to disconnect everything from the secondary
(or secondaries) of the power transformer. When you do so, the transformer
should drive no external 1o at all. Next, plug in the device and wait 10 to
20 minutes. Feel the case. Is it abnormally warm? Some heat is to be expected,
but if it’s abnormally warm under these conditions, the power trans former
is probably shorted. If the case is too hot to touch comfort ably, there is
definitely a problem with the power transformer.

You’ve narrowed down the problem to the power trans former, since nothing
else is getting power, so no other component can be the source of the excessive
heat. Nor can the transformer be overloaded, since it’s running with no load
at all, so the current drawn from its secondary winding(s) should be zero.
The only possible cause for this heating is some shorted windings within the
transformer itself. The transformer must be replaced or rewound.

You can use a dynamometer-type wattmeter for a more exact test. A good power
transformer with no load should give a very small rating, only 5 W or less
in a large power transformer. This small wattage is a result of the normal
iron loss effects within the transformer, which is why some heat is inevitable.
A higher watt age reading with no load is a clear indication of trouble.

Not all wattmeters will work properly for this test. You must use a dynamometer
type, with both a voltage coil and a current coil. Such a wattmeter will have
four terminals, instead of just two. If you don’t have a suitable wattmeter
handy, you can make a simple, crude, but reasonably effective wattmeter with
an ac voltmeter (the ac volts section of your multimeter) and a 1 power resistor
in series with the ac circuit to be monitored.

Checking the turns ratio of a transformer:

A power transformer consists of two coils closely wound around a single core.
Passing an ac voltage through one coil (the primary) will induce a proportionate
ac voltage in the other coil (the secondary). Note that some transformers have
multiple secondary windings with a single primary. The induced voltage across
the secondary winding might be the same as the input voltage applied to the
primary winding, but usually it will be different. The relationship between
the input and output voltages is determined by the relative number of turns
in each winding. There are three basic combinations:

The turns ratio is defined by the number of turns in the primary winding divided
by the number of turns in the secondary winding. Since this ratio will correspond
to the proportional volt ages, you can also define the turns ratio in terms
of the primary and secondary voltages:

Turns ratio = Primary voltage / Secondary voltage

Let’s see how this works with a few quick examples. In all cases, assume an
input (primary) voltage of 120 V.

First, let’s say the output (secondary) voltage is 24 V. This is a step-down
transformer with a turns ratio of:

Turns ratio = 120 / 24

= 5 greater

The turns ratio of a step-down transformer is always greater than 1.

For the second example, the output (secondary) voltage is also 120 V. This
is an isolation transformer. The turns ratio in this case is:

Turns ratio =120/120

=1

The turns ratio of an isolation transformer is always exactly 1. Finally,
let’s look at a step-up transformer with an output (secondary) voltage of 500
V. This time the turns ratio works out to:

Turns ratio =120/500

= 0.24

The turns ratio of a step-up transformer is always less than 1. In practical
electronics work, when you run across a trans former that may or may not be
bad, it’s helpful to know either the transformer’s rated primary and secondary
voltages or the turns ratio. (In the United States, it’s generally safe to
assume that the primary voltage of a power transformer is probably 120 Vac.)
If just a few turns are shorted in one of the transformer’s windings, it might
look fine using the resistance test described earlier in this section, but
there could still be a significant difference in secondary voltage for the
rated primary voltage because the turns ratio has been altered.

As an example, let’s use the 120 V: 24 V step-down trans former from an earlier
example. You already know the turns ratio of this transformer is 5. For every
turn in the secondary winding, there are five turns in the primary winding.
Assume there are 100 turns in the primary winding. This means there must be
20 turns in the secondary winding.

Now, let’s say four turns in the secondary winding are shorted. This will
remove less than an inch from the effective length of the conductor, so the
difference in the resistance probably will be negligible. It may even be undetectable.
However, the turns ratio of the transformer has been changed. Instead of 100/20
(5), the new turns ratio is:

Turns ratio =100/16

= 6.25

By rearranging the turns ratio equation, we can prove that the output (secondary)
voltage of the transformer is equal to the input (primary) voltage divided
by the turns ratio. That is:

Secondary voltage = Primary voltage/Turns ratio

=120/6.25

= 19.2 V

The output (secondary) voltage of this transformer will be too low because
of a few shorted turns in the secondary winding.

Fortunately, it’s not too difficult to perform a direct voltage test on a
transformer. First, remove the ordinary circuit load. Place a resistor across
the secondary winding to serve as a simple known load. To determine the proper
resistor value, you need to know the intended output (secondary) voltage and
the approximate current drain of the normal load. You can often get a good
estimate current value from the circuit’s fuse rating.

Once you know these factors, at least approximately, just use Ohm’s law to
determine the value of the load resistor.

Be careful. In most cases you cannot use a standard 1/2 W or ¼ W resistor.
You will probably need a larger power resistor. To determine the required wattage
rating, just multiply the voltage and the current:

P=EXI

= 24 X 0.05

= 1.2 W

In this example, the load resistor must have a wattage rating greater than
1.2 watts.

The next step in the test procedure is to measure the actual input (primary)
voltage. Sometimes ac power lines might run a little high or, more commonly,
a little low. Of course, this difference will affect the secondary or output
voltage. For example, if a transformer is rated for an output of 24 V with
an input of 120 V (turns ratio = 5), but the actual input voltage is only 110
V, the actual secondary voltage will be reduced to:

Secondary voltage = Primary voltage / Turns ratio

=110/5

=22V

Now, measure the voltage drop across the load resistor, as illustrated in
Fig. 9. Do you get the expected secondary voltage? If you get a reading more
than about 10 percent off from what it should be, the transformer is probably
defective.

_ Fig. 9 This is the test hook-up for checking the turns ratio of a transformer.

Rewinding transformers:

Usually, when a transformer tests bad, you will be inclined to simply discard
it and replace it with a new transformer of the same specifications. Unfortunately,
it’s sometimes difficult to find an exact replacement for a specific transformer.
Occasion ally, you will run into a problem with physical size. The available
replacement transformer might have the same electrical specifications as the
original unit, but if it has a larger body, it might not fit into the available
space in the equipment being serviced.

Even if you do find a suitable replacement transformer, these devices are
generally rather expensive. In some cases, it might make good economic sense
to try to repair the defective original transformer by rewinding it, rather
than simply replacing it. An other reason you might need to rewind a transformer
is if you can’t find one with quite the required turns ratio. By rewinding
the secondary, you can redesign the transformer’s specifications to suit your
requirements. To aid you in such repairs, this section will discuss the procedure
for rewinding a transformer.

Note that rewinding a transformer is a very picky, time- consuming job. It
takes a lot of patience, good concentration, and a steady hand. Many electronics
technicians simply aren’t temperamentally suited for such work. For them it
would always be worth the extra expense to seek out and purchase a replacement
transformer of the required type. Before attempting such a repair to save on
the cost of a new transformer, you should also consider what your time is worth.

We will only cover the procedure for rewinding the secondary winding of a
transformer. It’s possible to rewind a trans former’s primary winding, but
this procedure is almost always more trouble than it’s worth.

Try to avoid attempting such repairs on transformers with multiple secondary
windings. The design of the specific transformer in question also will help
determine the suitability of such a repair attempt. In some transformers, the
secondary winding is on top of the primary winding. Such transformers are usually
re-windable. Other transformers, however, have the primary winding on top of
the secondary winding. This arrangement is more common in step-up transformers
than in step-down transformers. Repairing this type of device would be excessively
difficult and time-consuming. It probably would not be worthwhile.

When attempting to rewind a transformer, it’s vital to work slowly, carefully,
and gently. Never use force.

If you intend to change the turns ratio, you must use a transformer with the
correct core size. The more power passing through the transformer, the larger
the core must be. A chart for roughly determining the cross-sectional area
of a transformer core is given in Table 6-1. Of course, if you are reducing
the secondary voltage, there should be no problem. These cores sizes are the
minimum for each wattage level.

Electrically, it doesn’t matter if the transformer’s core is too big, although
the device will be physically bulkier than it needs to be. The key point is
that you cannot reasonably expect to make too large a change in the original
transformer’s output power. For example, if the transformer was originally
rated for 24.6 V at 1.5 amps, you should be able to rewind it for 30 Vat 1
amp, or 18 V at 1.75 amps, but not 35 Vat 3 amps. The core would be too small,
and the transformer would burn itself out and possibly damage other components
in the circuit. It also might be a fire hazard.

__Table 1 The cross-sectional area of a transformer’s core limits its power-handling
capability.

Cross-sectional area in square inches | Maximum power in watts

1.00 --> 45

1.25 --> 50

1.50 --> 65

1.75 --> 75

2.00 --> 120

2.25 --> 150

2.75 --> 230

3.00 --> 275

3.25 --> 330

3.75 --> 440

4.00 --> 520

To begin the rewinding process, you must first open up the transformer. Remove
all screws or anything else holding the transformer together. The body of the
transformer (the core) is made up of multiple sheets of laminated metal. During
manufacture, these laminations were soaked in a special enamel and then baked,
so they are tightly sealed to one another. The enamel protects the transformer
from environmental contamination and helps prevent annoying transformer buzz.
Unfortunately, it also makes it difficult to remove the laminations.

You must individually break the coat of enamel holding each lamination in
place. Be very careful when doing so. If you use too much force, your tool
could slip and damage the transformer’s wires. Too much damage to the wrong
wires could render the transformer unrepairable.

You are likely to damage a few of the laminations themselves, especially
the first few, which are almost always the hardest to remove. They have the
most enamel, since they are on the out side. If you ruin a few laminations,
don’t worry too much about it. Because you will be reassembling the transformer
by hand, all the original laminations won’t fit back into place anyway. Just
try to work as carefully as possible and not damage too many of the laminations.

The best way to remove the first few laminations is to use a small screwdriver.
Attempt to work the blade between the top most lamination and the next one.
A few gentle taps with a hammer can help push the screwdriver’s blade between
the laminations. Don’t use hard blows. Be aware that this process is likely
to damage the blade of the screwdriver, so don’t use an expensive, precision
tool. A small, cheap screwdriver is not hard to find and will do the job. The
blade should be as narrow as possible.

Work the blade gently back and forth between the laminations until you manage
to break them apart. This procedure is difficult until you get the hang of
it.

After you have removed a few laminations, you will almost certainly come across
some that are I shaped. An I-shaped lamination will do a better job than the
screwdriver, so make the switch as soon as you can.

After removing the laminations, you can unwind the secondary of the transformer.
Do so slowly and carefully, at a time and place where you are unlikely to be
disturbed in the middle of the job. Be careful not to break or tangle the wire,
especially if you in tend to reuse it in the new winding. Keep track of the
number of turns in the secondary winding. To rewind the transformer for the
original specifications, you will need to use exactly the same number of turns.
If you intend to change the turns ratio, you will still need to know how many
turns there were so you can perform the necessary conversion.

If you are repairing a transformer with shorted windings, you should not attempt
to reuse the original wire in the new secondary winding. Use enameled wire
of the exact gauge as the original. You also will need to use new wire if the
new secondary winding calls for more turns than were used in the original transformer.

To determine the number of turns required for a different secondary voltage,
determine the original number of turns per volt. Just divide the total number
of turns in the original secondary winding by the original output voltage:

TV = To / Eo

where TV is the number of turns per volt, To is the total number of turns
in the original winding, and E is the original secondary volt age. As an example,
let’s say you have a 24.6 V transformer with 125 turns in its original secondary
winding. The turns per volt for this transformer is:

TV=125/24.6

= 5.0813

Now, if you want to rewind this transformer for a new secondary voltage of
18 V, just multiply the desired voltage by the turns-per-volt value:

NT = En X TV

= 18 X 5.0813

= 91.4634

where NT is the number of turns required for the new secondary winding, En
is the desired output voltage, and TV, of course, is the turns-per-volt value
derived previously.

Don’t worry too much about the fractional turns. In this ex ample, 91.5 turns
would certainly be close enough. Actually the difference between 90 or 91 turns
and 92 or 93 turns will be negligible. Make the new winding as close as possible
to the calculated number of turns, but don’t be obsessive about it.

The current-handling capability depends on the wire’s cross-sectional area.
To use the transformer for more than its originally rated current-handling
capability, you might need to use a heavier gauge wire. I don’t really recommend
doing so.

Inmost cases, you should use new enameled wire for the new secondary winding.
The original wire might have physical strain and kinks as a result of being
tightly wound so long, then being unwound and rewound. There also might be
chips in its insulating enamel, which could eventually result in shorted turns.
If you were very careful in unwinding the original coil, however, you might
be able to reuse the original wire, if you wish. Don’t reuse the original wire
if you are repairing a defective transformer, especially one with shorted turns.
You’ll just be rebuilding a defective transformer.

Wrap the wire around the core as closely as you can. Don’t leave any space
between turns so you get the maximum number of turns in the minimum amount
of space. We guarantee you won’t be able to get the windings as tight as they
were in the original manufactured unit, so your rebuilt transformer is likely
to be a little bulkier. This is okay.

When you finish one layer of turns, put some kind of insulation over it before
you begin the next, overlapping layer. You can use wax paper, electrician’s
tape, or any other thin, insulating material, provided that it’s capable of
withstanding the maxi mum voltage of the winding. Apply this insulating material
as tightly and as wrinkle- and bubble-free as possible to prevent excessive
bulk.

Once you have completely wound the new secondary, you must reassemble the
transformer’s laminations. Some will be E shaped; others will be I shaped.
They fit together as illustrated in Fig. 10. Because you are working by hand,
and the trans former was originally machine-assembled, it is-almost a sure
bet that you’ll end up with a few laminations left over—typically about two
to four of each type (E and!) if you are good at such delicate work. If you
can’t fit these extra laminations back into the transformer’s core, just discard
them and don’t worry about it. These omitted laminations will lower the power
rating of the transformer slightly, but the difference is not likely to be
significant or even noticeable. Of course, if you end up with a dozen or more
laminations left that won’t fit back into the core, you better de-rate the
power-handling capability of the rebuilt transformer somewhat.

_ Fig. 10 The laminations of a transformer’s core fit together like a simple
puzzle.

Finally, replace all the original screws and other hold-down devices that
you removed from the original transformer.

==Testing integrated circuits, modules, and PC units==

Printed-circuit units are appearing in radios and TV sets in large numbers.
These devices range from a simple RC integrator used in vertical sync circuits
to the equivalent of a whole amplifier circuit, each in one sealed package.
Admittedly, these units are impossible to check in detail because you can’t
get into them to test individual parts. However, there is at least one reliable
check you can do: an output check.

In the integrated circuit, check to see that the proper composite sync signal
is present at the input. If it’s not found at the out put, the unit is probably
bad. This method can be used on any module. Three things must be carefully
checked before any printed-circuit unit is condemned: the supply voltages and
cur rents; the input signal; and the output signal.

For instance, if the modular circuit is an audio amplifier, it will need a
certain amount of dc voltage supply and draw a certain amount of current. With
0.5 V of audio signal on the input, it has a normal output of 5 V. If the unit
meets these specifications, look elsewhere for the trouble. Don’t replace units
at random. Make definite tests and be positive before you replace any units.

The scope and signal generator can tell you if a stage is definitely bad by
checking input vs. output. If it’s bad, don’t overlook the supply voltage.

==A quick check for microphones==

There’s a good, quick check for almost all microphones, especially the common
dynamic and crystal types: Make them talk, rather than listen. Any microphone
can reproduce sound, as well as pick it up. For instance, if you have a tape
recorder that won’t record, the first question is whether the trouble is in
the mike or the amplifier. Feed an audio signal into the mike and listen.

You can use an audio-signal generator or any audio signal from a radio or
TV set. It takes only a very small signal to make a mike talk. A dynamic microphone
is nothing but a specially built dynamic speaker. Crystal mikes won’t talk
as loudly as dynamics, but even the variable-reluctance types used in communications
work will talk.

Incidentally, this is a good quality check for microphones if the complaint
is distortion in the sound output of a PA system or transmitter. By feeding
a music or voice signal into the mike and listening to it, you can detect dragging
voice coils, buzzes, etc. — defects that would distort the sound pickup. Also,
if you happen to have a replacement cartridge for the type of mike you are
testing, you can easily make A-B comparison tests of the sound quality of each.

This test can be reversed, too. If you have a complaint of possible mike trouble,
hook up a small dynamic speaker to the mike input and talk into it. If the
mike input is high impedance, use an output transformer to bring the low voice-coil
impedance up enough to work. Actual high-impedance mike transformers will be
up around 50,000 1 but you can use almost any output trans former. One of the
old 25,000 transformers is good, but the test will work with even a 10,000
1 type.

If you’re checking for possible mike distortion, hold the mike at least 10
to 12 inches away as you talk. You’ll be surprised at the quality of the sound.
Transistor radio speakers make good test mikes because of their tiny size.

==Checking phono cartridges with the scope==

When you have low gain in record-playing systems, one of the first things
to determine is whether the trouble is in the cartridge or amplifier. The scope
is a quick check for the cartridge. With its high-gain vertical amplifier,
you can use it as a sensitive ac volt meter.

Put a single-tone test record on the turntable, disconnect the cartridge leads
at the amplifier (although this isn’t really necessary if they’re soldered
in), and hook the vertical input of the scope to the hot wire, as shown in
Fig. 11. Set the vertical gain control of the oscilloscope to give about 1
inch deflection for a 1 V p-p input. Put the stylus on a band of continuous
tone — say 400 Hz. For the average crystal cartridge, the output will be from
1 to 3V.

_ Fig. 11 An oscilloscope can be used to check a phono cartridge. Test record
on turntable; GND; Scope

If you want to make a frequency run on the cartridge, you can do it even with
a narrow-band scope. Most of these scopes will go up to at least 50 kHz without
trouble. You’ll need a test record with a band of all frequencies on it, starting
at 30 Hz and going to 20 to 30 kHz, at the same output level. Several test
records of this type are available. You also can use this test on the whole
amplifier.

One valuable application of a scope test is checking stereo cartridges for
equality of output in the two channels. Use a test record with a monaural band
at about 400 Hz or a stereo band with equal outputs in the two channels. Several
test records have this band for checking speaker phasing, channel balance,
etc. Just read the output from each side of the cartridge; the two should be
the same.

Cartridge tests also can be made with an ac VTVM. The readings will be the
same as with the scope: 1 to 3 V p-p. Even an AC- VOLTS scale on the VOM will
do, although the meter must have a sensitivity of at least 10,000 per volt
on ac. The low input impedance of the VOM reduces the readings. They average
from 0.3 to 0.4 V, where the scope reads 1 to 3V. Crystal and ceramic phono
cartridges should work into a load impedance of 3 to 4 M.

==Impedance checker for speakers==

Impedance, or ac resistance, is generally difficult to measure. It’s nowhere
near as straightforward as dc resistance. For dc resistance, 100 is 100 , and
that’s that. It doesn’t matter what signal is flowing through the resistive
component. Impedance, on the other hand, is frequency-dependent. A component
that has a 100 impedance for a signal of 200 Hz might have an impedance of
38 when the signal frequency is changed to 500 HZ.

For speakers, the situation is further complicated because the intended signal
is a complex combination of multiple- frequency components. To make some sort
of comparison possible, a standard test signal of 1 kHz (1,000 ohms) is assumed
in de fining the impedance of a speaker. The test signal is a pure sine wave,
to avoid confusion from harmonic-frequency components.

The circuit shown in Fig. 12 is designed to test speaker impedance by comparing
the speaker being tested with a known 8 ohm speaker. A suitable parts list
for this project is given in Table 2.

IC1 and its associated components (R1 through R5 and C1 through C3) form a
simple sine wave oscillator. The component values suggested in the parts list
will give a signal frequency very close to 1 kHz.

Fortunately for this type of testing, high precision in the signal frequency
isn’t required. This simple circuit’s frequency will be close enough for our
purposes. If you prefer, you can replace this sine wave oscillator circuit
with almost any other sine wave oscillator. The nominal signal frequency should
be 1,000 Hz, and the signal amplitude should be between 3 and 10 V.

Fig. 12 This circuit can be used to test the impedance of loudspeakers.

When you first set up this circuit, you must calibrate the oscillator circuit.
Simply adjust trimpot R4 until you hear the clearest, purest tone from the
speaker. If you are a perfectionist, you can use an oscilloscope to monitor
the output of IC1. Again, adjust trimpot R4 until the oscilloscope shows the
most distortion-free sine wave.

This is a set-and-forget type of control. It might be a good idea to use a
drop of nail polish, glue, or paint to hold the trimpot’s shaft in place once
you have calibrated it to prevent the need for frequent recalibration. This
is a particularly good idea if you will move the tester around a lot.

The sine wave signal is fed through an impedance-matching transformer (Ti)
to give it a nominal impedance of 8 ohm. The impedance of the speaker under
test (connected to the circuit’s test terminals) is compared to the impedance
of the reference speaker (8 ohm). The two speakers to be compared are in a
bridge configuration, along with the two halves of potentiometer R6. This portion
of the circuit is redrawn as an equivalent circuit to give you a clearer picture
of the bridge. When the two halves of the bridge have an equal resistance (or
impedance), the oscilloscope will show a deep null.

You can easily make a calibrated dial for this tester by attaching known
small-valued resistors across the test terminals and marking the dial for the
null point. For example, you might start out with an 8 resistor. While watching
the oscilloscope, slowly adjust potentiometer R1 for the deepest null. Mark
the potentiometer’s position “8” on the dial. Repeat this procedure for resistances
to match each of the standard speaker impedances: 3.2 , 4 –ohm, 8–ohm, 16 ,
32 , 40 , and 100-ohm.

Now, when you connect an unknown speaker across the circuit’s test terminals,
you can adjust potentiometer R6 until you get the deepest possible null as
indicated by the oscilloscope. Read off the approximate value from your calibrated
dial. Some times you will get a speaker that falls between two standard calibration
points. That’s o.k. Just interpolate the value, or round it off to the nearest
standard impedance value.

If you don’t have an oscilloscope handy, you can substitute a pair of headphones
and listen carefully for the maximum audible null. This method is a little
less precise and requires more concentration. You also will have to make an
effort not to be distracted by the tones being produced directly by the two
speakers.

==Testing diodes==

You can test a semiconductor diode with an ohmmeter, but you must be careful.
Some semiconductor diodes are designed to handle only very small voltages.
Too high a test voltage could damage or destroy the diode you are attempting
to test.

To be safe, use an ohmmeter that uses a battery voltage of no more than 1.5
V. Some multimeters use up to a 9 V battery to power the ohmmeter section.
It’s a good idea to test unknown diodes on the highest range of your ohmmeter
because, at these higher ranges, the ohmmeter has greater internal resistance,
limits the current more, and puts less voltage across the diode connected to
the test leads. We’ve found that, for most common semiconductor diodes, the
upper resistance ranges give the easiest to read results anyway.

Measure the resistance across the diode from anode to cathode. Connect the
ohmmeter’s positive lead to the anode, and the negative lead to the cathode.
This arrangement forward-biases the diode. You should get a very low resistance
reading. The exact value will depend on the specific type of diode being tested.
Some diodes have a forward-bias resistance of about 1 to 10 1. Others might
have forward-bias resistances of just a few tenths of an ohm. A few diodes
might have higher forward-bias resistance.

Now, reverse the test leads. Connect the positive lead to the diode’s cathode,
and the negative lead to the anode. Now you should get a very high resistance
reading because the diode is re verse-biased. On some diodes, the reverse bias
might be just a couple hundred ohms, while others will exhibit resistances
well into the megohm (millions of ohms) range. Most semiconductor diodes will
have a reverse-bias range of at least several k-ohms.

It’s usually relatively easy to tell the anode from the cathode on a semiconductor
diode. Often a stripe around one end of the component’s body will indicate
the anode, as illustrated in Fig. 13. Some diodes have a tapered body at the
anode end, as shown in Fig. 14. Even if there is no visual indication at all,
however, this test procedure will indicate which is which, assuming the diode
is good.

(top) Fig. 13 On some diodes, the anode is indicated by a slope. Cathode, Anode
(tabove) Fig. 14 On some diodes, the body is tapered on one end to indicate
the location of the anode. Cathode, Anode

For quick, general-purpose go/no go diode testing, you just want to make sure
the diode exhibits a low resistance in one direction (when it’s forward biased)
and a significantly higher resistance when the applied test voltage has its
polarity reversed (the diode is reverse biased). If you get a low resistance
in both directions, the diode is shorted and should be replaced. If you get
a high resistance in both directions, the diode is open and should be replaced.
For the most part, a semiconductor diode is either good or bad. When it goes
bad, it’s almost always shorted or open, so this test is sufficient.

In some cases, you might be concerned with the exact specifications of the
diode. The most important factor is the front-to-back ratio of the diode, a
ratio of the forward-biased resistance to the reverse-biased resistance:

FBR= Rf/ Rr

where FBR is the front-to-back ratio, Rf is the resistance when the diode
is forward-biased, and Rr is the resistance when the diode is reverse-biased.

For example, if the forward-bias resistance is 75 ohm, and the reverse-bias
resistance is 12,500 (12.5K), the front-to-back ratio is equal to:

FBR = = 156.667

Is this reading good or bad? That depends entirely on the in tended specifications
for the diode in question. You must com pare your test results with the manufacturer’s
specification sheet for that type number, or you can make direct comparisons
with a diode of the same type that is known to be good. This example is probably
a good diode. Usually if there are problems, they will result in lower front-to-back
ratios.

==Replacing transistors==

It’s often difficult to find an exact brand-name replacement for a bad transistor,
especially one from an older, discontinued set. Foreign-manufactured equipment
tends to include a lot of unusually numbered parts.

Many component manufacturers offer a line of general re placement devices,
and all good technicians should have as many cross-reference guides as they
can find. Unfortunately, it’s not a good idea to place too much faith in any
cross-reference guide. The recommended replacements are always just close approximations,
not exact duplicates. A guide might list transistor A as a replacement for
transistor B, and it will probably work great in 99 percent of the circuits
using transistor B. But you may have the exceptional circuit. Also, don’t assume
that the replacements can work in either direction. That is, just because A
is listed as a re placement for B, don’t assume you could use B as a replacement
for A.

The best cross-reference guides are ones that include data and specifications
for each device. Generally, you will be most concerned with the voltage and
current ratings and the cutoff frequency. The other specs are also important,
of course, but in most applications they offer more room for error. When in
doubt, or if you run into problems, use a replacement transistor with slightly
higher ratings than the unit you are replacing.

==Derating components==

A component circuit designer should take the power-handling capabilities of
each component of the circuit into account. If the original component is rated
for 250 V, don’t replace it with one rated for only 100 V. Even if the circuit
works correctly with the substitution, the underrated replacement component
will be subject to premature failure. You’ll just need to replace it again
soon.

In some cases, the component originally installed by the manufacturer might
be significantly overrated because of parts availability or some other reason.
For example, we have seen ceramic disc capacitors rated for working voltages
of 500 V in pocket radios that operate off of a standard 9 V battery. It’s
highly unlikely that the component will ever see any voltage coming even remotely
close to 500 V in this circuit. In this case, you can safely substitute a capacitor
with a lower voltage rating.

Usually things aren’t quite so obvious. The general rule of thumb is to never
substitute a component with a lower voltage or power rating than the original
component unless you are absolutely sure that the manufacturer overrated the
component for an electrically irrelevant reason (such as parts availability
or cost). If there is any doubt, don’t make the substitution. You might just
be asking for trouble in the future.

On the other hand, you usually can overrate the voltage or power ratings of
your replacement components. For example, you can replace a 2.2K, 1/4 W resistor
with a 2.2K, ½ W resistor. All you will have to worry about is whether or not
the new, heftier component will physically fit.

In some cases, a manufacturer might have cut corners or the circuit designer
might have erred, and the original component might not be adequately rated,
or just barely so. If you service a number of units of a given piece of electronic
equipment and repeatedly come up against a specific capacitor being burnt out,
the odds are that its working voltage rating just isn’t good enough for the
operation of the equipment. In this case, you definitely don’t want to use
an exact replacement. Substitute a capacitor with the same capacitance value,
but with a somewhat higher working voltage rating. This replacement will often
give the equipment much greater reliability. We’re constantly shocked by how
often manufacturers use inadequately rated components, even in high-grade equipment.

As a rough rule of thumb, never expose any component to more than 75 to 80
percent of its maximum voltage or power rating during the normal operation
of the circuit. For example, never expect a capacitor rated for 250 V to handle
more than about 188 to 200 V, and a 0.25 W resistor shouldn’t have to carry
more than about 0.2 W in the normal operation of the circuit. This rule will
leave some headroom for the component to handle any unexpected transients or
overvoltage conditions without unnecessary damage.